This invention generally relates to radio transmitters. More specifically, it relates to apparatus and method for providing linear radio transmitters via cartesian loop circuitry.
Today""s radio communications systems typically operate in narrowly defined bands of frequency. Consequently, the transmitters in radio communication systems require power amplifiers that are capable of operating in a highly linear fashion to maintain the spectral efficiency, thereby minimizing the resulting interference.
The use of cartesian loop circuits to achieve linear power amplification in communication system transmitters is known in the art. A known cartesian loop circuit is shown in FIG. 1 and is described in an article entitled xe2x80x9cRF Linear Amplifier Design,xe2x80x9d by P. B. Kenington and A. Bateman, published in the Proceedings of RF Expo West at pages 223 to 232 in March of 1994.
The cartesian loop circuit of FIG. 1, however, has several limitations. One general limitation is the inability to provide adequate power control, a necessary function in most communication system transmitters. By way of example only, existing circuits do no provide an adequate range of power control. Further, existing circuits also do not provide adequate fine power control. There are other limitations in the power control offered by existing cartesian loop circuits.
Existing cartesian loop circuits also do not provide any noise filtering to remove the noise introduced by the cartesian loop. Thus, existing cartesian loop circuits provide noisy operation. Existing cartesian loop circuits also do not provide adequate calibration technique to compensate for DC offset of components and to compensate for phase variations in the cartesian loop. All of these limitations place a limit on the accuracy of operation.
Other limitations include the lack of instability detection, the lack of adaptive operation during overheat situations and during undervoltage situations and the lack of any type of detection of the transmission mask to ensure the proper operation of the cartesian loop transmitter.
In view of these and other limitations, new and improved cartesian loop circuitry is needed, particularly for use in linear transmitters in radio communication systems.
The present invention provides method and apparatus for nulling DC offsets in a feedback path of a cartesian loop that consists of a forward path and the feedback path, thereby minimizing degradation of the performance of the cartesian loop. In accordance with the method of nulling DC offsets, the operation of the forward path is disabled and the DC offset in the feedback path is sensed and stored. The DC offset in the feedback path is then subtracted out while the forward path is enabled. The downconverter is preferably driven with its local oscillator during this process since the local oscillator can affect the DC offset. In accordance with a preferred embodiment, if system timing constraints do not permit the local oscillator to lock at its specified stability, the storing of the DC offset is done when the local oscillators have reached a predetermined stability.
The DC offset nulling is preferably performed with a sample and hold that senses and acquires the DC offset in the feedback path when the forward path is disabled. The forward path is disabled preferably by disabling a power amplifier in the forward path.
In accordance with another aspect of the present invention, apparatus and method for filtering out the noise in a cartesian loop circuit are provided, thereby improving full duplex operation in a communication system using the cartesian loop transmitter. In accordance with the apparatus of this aspect of the present invention, a noise filter is provided in the forward path that blocks frequencies outside the frequency band over which signals are transmitted. The filter is preferably a bandpass filter having a bandwidth approximately equal to the frequency band of the signal to be transmitted. Thus, if the cartesian loop is being used in a frequency hopping systemxe2x80x94wherein a plurality of frequency channels are usedxe2x80x94the filter has a bandwidth that allows transmission over all frequency channels. Importantly, the noise filter blocks frequencies generated by the components in the cartesian loop that are within the band of frequencies used to receive signals.
In accordance with another aspect of the present invention, apparatus and method for detecting when a cartesian loop is operating in an unstable fashion is provided. In accordance with the method of this aspect of the present invention, the baseband signals in the cartesian loop are monitored to detect when they have a frequency outside the bandwidth of the baseband input signals. When a frequency outside the bandwidth is detected, the operation of the cartesian loop is controlled to try to restore stable operation or to minimize the possibility of the breaking of the transmission mask.
In accordance with another aspect of the present invention, apparatus and method for detecting undervoltage and over-heat operation are provided. To detected over-heat operation, the temperature at which the cartesian loop circuit operates is detected. When the temperature exceeds a first threshold, the power output of the cartesian loop circuit is lowered. When the temperature exceeds a second threshold, operation of the cartesian loop circuit is stopped. Alternatively or in addition, the gain of the loop can be differentially controlled or the phase of the loop can also be controlled. These actions can be taken in any order with any priority.
To detect undervoltage operation, the power supply voltage supplied to the cartesian loop circuit is detected. When the voltage falls below a first threshold, the power output of the cartesian loop circuit is lowered. When the voltage falls below a second threshold, operation of the cartesian loop circuit is stopped. Alternatively or in addition, the gain of the loop can be differentially controlled or the phase of the loop can also be controlled. These actions can be taken in any order with any priority.
In accordance with another aspect of the present invention, apparatus and method for controlling the phase of a cartesian loop having one or more power control components is provided. In accordance with the method, the on/off status of each power control component in the cartesian loop is determined. Then, the phase of the cartesian loop is adjusted depending on which components are in the loop. This adjusts for the varying phase delays caused by components in the loop. In accordance with another aspect of this invention, when the cartesian loop is used in a communication system that communicates over a plurality of frequency channels the frequency used for a communication is determined and the phase of the cartesian loop is adjusted in accordance with the frequency.
In accordance with another aspect of the present invention, when a cartesian loop transmitter is used in a time slotted communication system, apparatus and method are provided to control the transmissions at the start of an active time slot to allow the cartesian loop to reach full operation before full power transmission. In accordance with the method, the timing of an active time slot is determined. Then, the output from the cartesian loop during the start of the active time slot is delayed. This is preferably accomplished by ramping the power control signal to an output power amplifier in the forward path of the cartesian loop circuit.
In accordance with another aspect of the present invention, apparatus and method for simply controlling the power output of the cartesian loop transmitter are provided. In accordance with the method, one or more stages of a multi-stage power amplifier in the forward path of a cartesian loop are bypassed with a bypass switch when lower power output is desired. It is preferred to control the phase of the cartesian loop as stages are bypassed to account for the changes in loop phase.
In accordance with another aspect of the present invention, apparatus and method for controlling the power output of a cartesian loop by selectable loop attenuation with a PIN diode circuit are provided. In accordance with the apparatus, the forward path of the loop includes a first input for receiving baseband signals, a second input, means to upconvert the input signals to a signal to be transmitted, an attenuator having selectable attenuation and a plurality of power amplifier stages. The feedback path includes an input from the forward path, a PIN diode attenuator having selectable attenuation and means to downconvert and an output that provides a feedback signal to the second input. It is preferred to control the attenuation so that the attenuation removed in the forward path equals the attenuation added in the reverse path and vica versa.
In accordance with a further aspect of the present invention, apparatus and method that allow selectable control of the output power of the loop are provided. In accordance with the apparatus, the loop includes a forward path having a first input for the baseband signals, a second input, means to upconvert the input signals to a signal to be transmitted, an attenuator having selectable attenuation, an amplifier having selectable amplification and a plurality of power amplifier stages. The feedback path includes an input from the forward path, an attenuator having selectable attenuation, an amplifier having selectable amplification and means to downconvert the input from the forward path. Again, it is preferred to match gain control in the forward path and in the feedback path.
In accordance with yet another aspect of the present invention, apparatus and method for providing fine and coarse control of the output power of the cartesian loop is provided. The loop circuit includes means for controlling the gain of the baseband signals in addition to the gain control elements previously discussed. It is preferred to provide fine control of the cartesian loop power output by the means for controlling the gain of the baseband signals and to provide coarse control of the cartesian loop power output by controlling the gain control means in the forward and feedback paths.